My best results took me from weighting 135ish up to 153 over the course of about 5 months by adding a quart of chocolate milk as my post training meal and following stronglifts 5x5.
Once I got close to my strength goal I switched to a more Paleo diet, but kept lifting. My weight went back down around 140 but my strength stayed.

Here's another vote for squats, deadlifts, and milk. Personally I prefer Starting Strength (particularly the book, not the wiki) over StrongLifts, but the core is the same. Lifting heavy and eating big makes bigger and stronger.

What a disgrace it is for a man to grow old without ever seeing the beauty and strength of which his body is capable. -Xenophon's Socrates

This is tangential to the topic of the thread, but why would anyone want to gain weight longterm unless they were medically underweight or had an eating disorder? I've even read that many people at a "healthy weight" are actually still too fat.

This is tangential to the topic of the thread, but why would anyone want to gain weight longterm unless they were medically underweight or had an eating disorder? I've even read that many people at a "healthy weight" are actually still too fat.

Because for most of my life I've been 5'8" and 115 lbs and I'd like for once to be able to buy a pair of pants that fucking fit.

I've even read that many people at a "healthy weight" are actually still too fat.

That part's true. Weight is a very crude measure; what you really want is body fat percentage or fat free mass. Muscle is denser than fat. There are plenty of "skinny-fat" people who fall into the "normal" range for height to weight (however that's derived...) but have high body fat. On the other other hand, many athletes or heavily muscled people are overweight by scale measure, but have low body fat. Also, BMI is an almost worthless measure for individuals.

Edit: Skinny guys really need to get over visible abs thing if they want to get bigger. You almost have to add fat along with the muscle and then sometime later (MUCH later-years if you do it right) diet back down. The trick it to not add too much fat compared to muscle. It's called "bulking."

That part's true. Weight is a very crude measure; what you really want is body fat percentage or fat free mass. Muscle is denser than fat. There are plenty of "skinny-fat" people who fall into the "normal" range for height to weight (however that's derived...) but have high body fat. On the other other hand, many athletes or heavily muscled people are overweight by scale measure, but have low body fat. Also, BMI is an almost worthless measure for individuals.